1. Field of the Invention
The present invention is directed to a method for correcting a calibration table of a (CT) apparatus that contains existing calibration values, the CT apparatus having a detector system formed by Nxe2x89xa72 rows of detector elements following one another in the z-direction, that include a first active row of detector elements in the z-direction and a last active row of detector elements in the z-direction.
2. Description of the Prior Art
In such CT apparatus, each detector channel has its own sensitivity, i.e., the electrical output signals of the respective channels are usually different in amplitude given the same incidence of X-ray quanta per channel. As used herein xe2x80x9cdetector channelxe2x80x9d means the signal path from the detector element to the digitalization point where conversion to digital form occurs.
One purpose of calibration is to identify the individual sensitivities of the channels and to store corresponding calibration values in tablesxe2x80x94the calibration tablesxe2x80x94for later corrections of the measured values. Attenuation values would be undefined without these corrections. Since the calibration values represent the reference values of the radiation intensities without an attenuating subject, the calibration tables are also referred to as air tables or air calibration tables. Without calibration, the tomograms would be covered by pronounced ring artifacts.
Such calibration tables are required dependent on parameters such as slice thickness B, tube voltage U, rotation time T, a switchable pre-filtering, possibly two currents i and detector temperature xcex8.
The number of possible parameter combinations of B, U, T and, possibly, i, xcex8 is extremely high, so that the outlay for generating and storing corresponding calibration tables would be considerable. Moreover, all calibration tables would have to be updated with corresponding measurements at every re-calibration of the system xe2x80x94and such a recalibration can be necessary daily.
For time reasons, a separate calibration table is not produced for every individual parameter combination in a CT apparatus. It is conventional, for example, to produce a base table for each slice thickness B, this being calibrated daily. The other parameters are usually set to average values. The table that is utilized in all other parameter combinations is derived from the addition of a base table of the slice thickness B that has been set and one or more difference tables that contain the deviation relative to the modified parameters. The difference tables then need not be calibrated daily but only once in the factory or given a hardware replacement (for example, installation of a new x-ray tube).
In a single-line CT apparatus (detector with one line or row of detector elements), for example, the table for the parameter combination slice thickness B=1 mm, voltage U=140 kV, rotation time T=1 sec is composed of a base table for B=1 mm, U=120 kV and T=0.75 sec and of a voltage difference table for B=1 mm, U=140 kV and T=0.75 sec, as well as of a rotation time difference table for B=1 mm, U=120 kV and T=1 sec.
The above method in fact still requires some time but delivers good results for single-line CT apparatus.
In a multi-line CT apparatus, however, the problem arises that no accurate calibration is possible with respect to the outer rows, particularly with respect to the two outermost, active rows of detector elements, i.e. the first active row in the z-direction and the last active row in the z-direction.
An object of the present invention is to provide a method of the type initially described which enables an accurate calibration of a multi-line CT apparatus.
This object is inventively achieved in a method wherein the calibration values contained in the calibration table T(n,k) with respect to the outer active rows of detector elements are corrected by producing a reference vector R(k), and on the basis of this reference vector errors F(n,k) of the calibration values with respect to the outer active rows of detector elements are identified, and corrected calibration values Tcor(n,k) with respect to the outer active rows of detector elements are acquired by subtracting the identified errors from the corresponding calibration values. The calibration table T(n,k) can be a base table TB(n,k) or a difference table TD(n,k).
The inventive method therefore enables an improved calibration of a multi-line CT apparatus because the calibration measurements for a multi-line CT apparatus in the outer, particularly in the two outermost, active rows are affected by time-variant errors that, for example, occur due to a non-reproducible diaphragm positioning, particularly of the detector-proximate diaphragms, due to aging processes or due to temperature effects.
When the calibration is based on a base table TB(k) and m=2 difference tables TD1(k) and TD2(k), the following applies ideally for an arbitrary row of detector elements for the corrected signal S(k) acquired from the measured signal M(k):
S(k)=M(k)xe2x88x92(TB(k)+TD1(k)+TD2(k)), whereby 
S is the corrected signal,
M is the measured signal of a channel,
TB is the base table,
TDm: is the difference table m, and
k is the channel index.
Since, however, each measurement, i.e. the production of tables, is affected by the time-variant error F(t,k) (t is time), the following in fact applies:
S(t,k)=M(k)+F(t,k)xe2x88x92(TB(k)+F(t3,k+TD1(k)+F(t1,k)+TD2(k)+F(t2k)) 
or, in a first approximation:
S(k)=M(k)xe2x88x92(TB(k)+TD1(k)+TD2(k))xe2x88x922F(t2,k), 
i.e. the sum of the errors of the difference tables employed remains in a first approximation. This is true because the base table is in fact measured on the same day as the measurement itself and (t≈t3)xcx9cF(t,k)≈F(t3,k) therefore applies. The difference tables, however, are measured together during the manufacture in the factor (t1≈t2), for which reason F(t1,k)≈F(t2,k) applies.
Whereas the time-variant errors are negligible for inner active rows, the time-variant errors of the outer, particularly of the outermost, active rows, which would produce losses in image quality, are corrected by the inventive method,